1. Field of the Invention
The present invention relates to a mobile communications control method and control system for controlling mobile communications by mobile terminals in a network in which a plurality of wireless systems are available, and a mobile terminal used therein.
2. Description of the Related Art
Wireless systems conventionally known include cellular systems such as Personal Digital Cellular (PDC) Telecommunication Systems and International Mobile Telecommunications-2000 (IMT-2000), wireless LAN systems such as IEEE802.11a/b, and Bluetooth (registered trademark).
For higher-speed and larger-volume communications in cellular systems, research and development on fourth-generation systems (4G cellular systems) has been conducted. For wireless LAN systems, standardization on IEEE.802.11g and the like has been conducted. Also, research and development on new wireless systems such as Ultra Wideband (UWB) has been conducted. Given these situations, wireless systems will be increasingly diversified in the future.
FIG. 1 illustrates a Beyond IMT-2000 mobile communications system in which service areas LA_A to LA_F of multiple wireless systems A to F overlap one another in a single physical area.
It is envisioned that the Beyond IMT-2000 mobile communications system will include a multimode mobile terminal 10 capable of mobile communications in a wireless system A, Bluetooth system B, wireless LAN system C, 4G cellular system D, 3G cellular system E, wireless system F and so on.
In this context, development on multimode mobile terminals compliant with multiple wireless systems has also been actually promoted. PHS/PDC dual terminals have already been developed. PDC/IMT-2000 dual terminals, IMT-2000/IEEE802.11b dual terminals and so on are expected to appear in the future. Study on software radio technology has also been actively conducted.
As shown in the following non-patent documents 1, 2, 3, and 4 and patent document 1, study on location registration control (location control) in networks having a plurality of wireless systems has conventionally been conducted.    [Non-patent Document 1] Qing Tian, Donald C. Cox (Stanford Univ.), “Location Management in a Heterogeneous Network Environment”, IEEE Wireless Communications and Networking Conference (WCNC) 2000.    [Non-patent Document 2] Gang Wu, Mitsuhiko Mizuno (Communication Research Laboratory), Paul J. M. Havinga (University of Twente, Netherlands), “MIRAI Architecture for Heterogeneous Network”, IEEE Communication Magazine, February 2002.    [Non-patent Document 3] Yi-Bing Lin (National Chaio Tung Univ.), Imrich Chlamtac (Boston Univ.), “Heterogeneous Personal Communications Services: Integration of PCS Systems”, IEEE Communication Magazine, September 1996.    [Non-patent Document 4] Mark Stemm, Randy H. Katz (U.C. Berkeley), “Vertical Handoffs in Wireless Overlay Networks”, ACM Mobile Networking (MONET), 1998.    [Patent Document 1] Japanese Patent Application No. H06-260990.
The technique disclosed in the non-patent document 1 focuses on matters related to a location management database, and the technique does not make detailed description on location registration control in networks having a plurality of wireless systems.
In the technique disclosed in the non-patent document 2, a wireless control system, called a Basic Access Network (BAN), commonly used by a plurality of wireless systems performs location registration control.
Therefore, there is a problem in that a wireless system to perform the location registration control of a specified mobile terminal is thus fixed, and it is impossible to choose an optimum wireless system with which the mobile terminal performs the location registration.
In the technique disclosed in the non-patent document 2, when a wireless system used for paging is different from a wireless system used for communication, it is necessary to activate an interface of the wireless system used for communication after paging, resulting in a possible delay in starting communication.
The technique disclosed in the non-patent document 3 integrates multiple Personal Communication Systems (PCSs), and envisages two cases, location registration by a mobile terminal with only one wireless system and location registration with multiple wireless systems. However, there is a problem in that no consideration is given to the choice of an optimum wireless system to perform location registration control of the mobile station.
The technique disclosed in the non-patent document 4 relates to a method of selecting an optimum wireless system during communication to switch a wireless system for use (Vertical Handoff). In this method, there is a problem in that a wireless system can be switched only during communication, and before initiating communication, a wireless system cannot be switched.
The patent document 1 discloses, as a method of selecting a wireless system by a multimode mobile terminal, a method of selecting a wireless system in a better radio environment, when the radio environment of a different wireless system is deteriorated.
Also, the patent document 1 discloses, as a method of selecting a wireless system by a multimode mobile terminal, a method of selecting a wireless system based on priority uniformly predetermined for all mobile terminals in a network in which multiple wireless systems are available.
The patent document 1, however, does not disclose a method of selecting an optimum wireless system for each mobile terminal.
Generally, different wireless systems have different characteristics and different advantages and disadvantages. For example, cellular systems excel in mobility management, area coverage and battery saving. On the other hand, they have difficulty in providing high transmission rates.
Wireless LAN systems excel in the provision of high transmission rates. On the other hand, they provide hot-spot-like area coverage, having difficulty in dealing with high-speed mobility of terminals, and are unfavorable in battery saving.
The above-described conventional techniques do not allow for controlling mobile terminal location registration in a network in which a plurality of wireless systems are available, effectively utilizing the characteristics of the plurality of wireless systems.
According to the above conventional techniques, when a service area LA_A of a first wireless system A overlaps a service area LA_B of a second wireless system B in a single physical area as shown in FIG. 2, all mobile terminals included in a mobile terminal group 10A perform location registration with either of the first wireless system A or the second wireless system B, or the mobile stations perform location registration with a wireless system for controlling location registration. There is thus a possibility of traffic concentration on a single wireless communication system.
Also, according to the above conventional techniques, a wireless system with which a mobile terminal performs location registration is fixedly selected. When there is an incoming call to the mobile terminal, calling is performed via the fixedly selected wireless system. There is thus a possibility in that an optimum wireless system as a wireless system for use cannot be selected at the start of communication.
In this case, there is a possible method of selecting a wireless system different from a selected wireless system at the start of communication. This method, however, requires a number of steps of selecting an optimum wireless system, securing a communications channel and so on, before actually initiating communication, resulting in a potential problem of delay before the start of communication.
Even when a wireless system providing a hot-spot-like service area such as a wireless LAN system is detected, a mobile terminal moving at a high speed is located in the service area for a short time. If the mobile terminal does not establish communication or does not finish communication while located in the service area, such a problem can occur that location registration should not have been performed with the wireless system.